1. [Field of the Invention]
The present invention relates to a heat sensitive flow meter for detecting the flow rate of a fluid using heat sensitive elements.
2. [Description of the Prior Art]
In an electronically controlled fuel injector for a car engine, it is important to measure the quantity of intake air for an engine for the control of air/fuel ratio with high accuracy. A heat sensitive flow meter is used in an air flow detector. Two heat sensitive elements for detecting the flow rate of a fluid such as air are formed on a ceramic substrate by winding a temperature sensitive resistor film made from platinum or a platinum wire. The temperature of the fluid is detected by a first heat sensitive element making use of changes in the resistance of this heat sensitive element caused by temperature variations and a current is supplied to a second heat sensitive element so that the temperature of the second heat sensitive element becomes higher than the temperature of the first heat sensitive element by a predetermined temperature. When the second heat sensitive element is cooled by the flow of the fluid, a current supplied to maintain the second heat sensitive element at a predetermined temperature increases, and the flow rate of the fluid is measured from this change in current value. This control system is called "fixed temperature difference control system" or "fixed temperature control system".
This heat sensitive flow meter in which the first and second heat sensitive elements and a plurality of fixed resistors constitute a bridge circuit and this bridge circuit is controlled by an operation amplifier is already known. There is also known a technology for controlling the responsibility and safety of this bridge circuit by adjusting the off-set voltage of this operation amplifier to a predetermined value.
For example, FIG. 10 is a circuit diagram showing an example of a conventional heat sensitive flow meter, and FIG. 3(C) is a graph showing the waveform of output B when a power voltage is applied to this conventional heat sensitive flow meter. In FIG. 10, a terminal T to which a power voltage is applied is connected to the connector of a transistor 8, the emitter of the transistor 8 is connected to one end of a first heat sensitive element 1 and to one end of a second heat sensitive element 2, the other end of the first heat sensitive element 1 is connected to one end of a fixed resistor 3, the other end of the fixed resistor 3 is connected to one end of a fixed resistor 4 and to the inversion input terminal of an operation amplifier 7, and the other end of the fixed resistor 4 is grounded. The other end of the second heat sensitive element 2 is the output (V5) of the bridge circuit and connected to one end of a fixed resistor 5 and to the non-inversion input terminal of the operation amplifier 7 through a DC offset voltage 33. The output of the operation amplifier 7 is connected to the base of the transistor 8, and a current is supplied from a power source to the bridge circuit through the transistor 8 to maintain the balance of the bridge circuit. The above first heat sensitive element 1 is connected to a bridge branch SA and the second heat sensitive element 2 is connected to a bridge branch SB. The first and second heat sensitive elements 1 and 2 are placed at predetermined locations on the above ceramic substrate.
Describing the operation of this heat sensitive flow meter, when the flow rate of air increases, the second heat sensitive element 2 placed in the flow of air is cooled and the resistance value thereof decreases, thereby increasing the potential of a connection point between the second heat sensitive element 2 and the fixed resistor 5. This voltage change raises the non-inversion input voltage of the operation amplifier 7, an output voltage thereby rises, a current is supplied to the bridge circuit through the transistor 8, the second heat sensitive element 2 generates heat with this current, and the temperature of the second heat sensitive element 2 is thereby increased to maintain a fixed temperature difference between it and the first heat sensitive element 1. Generally speaking, as the operation amplifier 7 has primary delay characteristics and the second heat sensitive element 2 has a thermal delay, a fixed temperature difference control circuit shows secondary delay characteristics. Since the DC offset voltage 33 is provided for the stable operation of the secondary delay system, the circuit can operate stably at the entire range of flow rate.
Heat generated from the second heat sensitive element 2 is transmitted to the air and to a support section for supporting the second heat sensitive element 2 and consumed as a loss. When a power voltage is applied, a heat transmission loss to this support section cannot be ignored and this heat transmission gradually changes over a long time. For example, when a power voltage is applied, as shown in the output B of FIG. 3(A), a flow signal shows a tendency to gradually reach a final flow rate from a flow rate a little higher than the final flow rate.
It is known that, when the flow rate sharply changes, responsibility is reduced by the influence of a heat transmission loss to the support section. As the prior art for improving responsibility, FIG. 11 shows a fixed temperature difference control circuit for a heat sensitive flow meter disclosed by Japanese Laid-open Patent Application No. 7-63588. When this circuit is compared with the circuit of FIG. 10, a differential circuit 34 is connected to the output of the bridge circuit comprising the second heat sensitive element 2 and the fixed resistor 5, and the output of the differential circuit 34 is divided into two and connected to comparators 35 and 36, the outputs of the comparators 35 and 36 are integrated and connected to a constant current circuit 37, and the output of the constant current circuit 37 is connected to the non-inversion input terminal of the operation amplifier 7. One end of a fixed resistor 6 is connected to the non-inversion input terminal of the operation amplifier 7 and the other end of the fixed resistor 6 is connected to the outputs of the second heat sensitive element 2 and the fixed resistor 5 to form a loop, and the output of a constant current circuit 38 is connected to the non-inversion input terminal of the operation amplifier 7. This constant current circuit 38 is driven by a voltage Vcc obtained by dividing a power voltage by unshown resistors and controlled by the feedback control of the loop to supply an offset voltage .DELTA.E.
When the flow rate sharply changes, the feed-back control is carried out according to a flow change signal to temporarily change this offset voltage .DELTA.E, thereby improving resposibility.
In the prior art, when the heat sensitive elements 1 and 2 generate heat by the application of a power voltage, they transmit heat to the support section and then become stable at a predetermined temperature. Therefore, it takes time to reach a target signal, thereby deteriorating stability at the time of application of a power voltage and causing an output error until the heat sensitive elements 1 and 2 become stable.
In a heat sensitive flow meter disclosed by Japanese Laid-open Patent Application No. 7-63588, responsibility is improved by temporarily changing the offset voltage .DELTA.E according to a flow change signal. However, since the heat sensitive elements become stable after they transmit heat to the support section, the influence of an output error at the time of application of a power voltage is not cancelled. In addition, when the heat sensitive elements 1 and 2 having low responsibility are used and the offset voltage .DELTA.E is temporarily changed according to a flow signal, the output of the bridge circuit readily oscillates due to feedback control, especially at the time of application of a power voltage.